How accurate is Inertial Earth Rotation Sensing utilizing Large Ring Lasers 

Ring lasers are now resolving the rate of rotation of the Earth with 8 significant digits. Technically they constitute a Sagnac interferometer, where a traveling wave resonator, circumscribing an arbitrary contour, defines the optical frequency of two counter-propagating resonant laser beams. Subtle non-reciprocal effects on the laser beam however, cause a variable bias, which reduces the long-term stability. Over the last two years, we have improved the performance of the G ring laser to the point, that we obtain long-term stable conditions over more than a year. Advances in the modeling of the non-linear behavior of the laser excitation process as well as some small but significant improvements in the operation of the laser gyroscope are taking us now right to the point, where the periodic part of the Length of Day variation of the Earth rotation can be recovered. Furthermore, we also extract the precession and nutation motion of the earth itself from the data as well. This corresponds to a rotation signal of 50 seconds of arc per year. It is the first time that this has been achieved by an inertial sensing technique. A laser gyroscope is a local sensor, but we extract a global quantity from it. How accurate are these measurements and where are the persisting error sources? This talk outlines the current state of the art of inertial rotation sensing in the geosciences and its remaining challenges. Furthermore, we discuss promising ways for a further enhanced sensor stability.